Explosive compaction of powders



Feb. 27, 1962 D. L. COURSEN ETAL 3,022,544

EXPLOSIVE COMPACTION OF POWDERS Filed Feb. 6, 1958 2 Shets-Sheet 1 F lG. I 8

F l G. 2

INVENTORS DAVID L. COURSEN GEORGE A. NODDIN JAMES I. REILLY ATTORNEYFeb. 27, 1962 D. L. COURSEN ETAL 3,022,544

EXPLOSIVE COMPACTION OF POWDERS Filed Feb. 6, 1958 2 Sheets-Sheet 2FIG?) INVENTORS DAVID LINN COURSEN GEORGE ADELBERT NODDIN JAMES ISIDOREREILLY BY v2; M

,, ATTORNEY 3 022,544 EXPLQSHVE CGMiPACTTON 6F POWDERS David L. Coursen,Newark, Deb, and George A. Noddin, Sewell, and James I. Reilly,Woodbury, N..I., assignors to E. I. du Pont de Nemours and Company,Wilmington, Del, a corporation of Deiaware Filed Feb. 6, i958, Ser. No.713,718 2- Claims. (Cl. 18-593) The present invention relates to theforming of compacts from powdered material. More particularly, thisinvention relates to a novel method for compacting powders wherebycompacts of unusually high density and strength are obtained and wherebylarge compacts can be produced.

This application is a continuation-in-part of our copending applicationserial No. 697,614 filed November 20, 1957, now abandoned.

The techniques of powder metallurgy are widely employed in theprocessing of metals melting at high temperature, sometimes referred toas refractory metals, and in the preparation of combinations of metalsthat cannot otherwise be obtained because the metals are not miscible orbecause of very wide differences in melting point. In essence, thetechnique involves compression of the loose metal powder into aconsolidated mass which can be further processed. In most cases, suchfurther processes involve first a sintering operation to make the masscoherent. In the case of many materials, the coherent mass thus preparedcan be rolled or drawn directly to the desired product. In other cases,the sintered compact must be further processed before the metal can befabricated into finished products. In either'case, the quality of thecompact is largely dependent upon the uniformity and degree ofcompression obtained in the first step. The strength of the sinteredcompact is directly dependent upon the density of the compact.

At present, most compacts of powdered metals are produced by introducingthe loose powder into a die and compressing it by a mechanical press.This operation poses many. problems. For example, the compressed metalwill have a density of from 3 to 15 times that of the loose powder; thusthe stroke of the press and the size of the die must be sufficient tocompensate for the reduction in volume occurring during compaction. Thisfactor 7 alone imposes a severe limitation on the size of the compactwhich can be prepared, since presses having very long strokes are notfeasible, particularly when very high pressures, i.e., up to 200 tonsper square inch, are required. The compacting load in a press isproportional to the area of the powder being compacted. Therefore, foran average compacting pressure of only 50 tons per square inch, a 500ton press could press a compact of only 10 square inches. Obviously, topress com-pacts of large surface area, enormous presses are required.When pressures of 200 tons per square inch are required and largesurface areas are involved, the mechanical press becomes unfeasible.Removal of the compact formed from the die represents a further problemwith some metals since the compact has a tendency to expand when removedfrom the die. Thus, as the compact is being removed, that portion of thecompact outside of the die expands while the portion within the diecannot; this action causes cracks in the compact. Provision must be madefor the escape of air from the die during the compression withoutpermitting the powder to be blown out. The die walls must be smooth tofacilitate removal of the compact and sufficiently heavy to withstandthe tremendous pressures involved. In addition, many powder compositionscontain hard and abrasive particles. When such compositions aremechanically pressed, the life of the die is very short and frequentreplacement is required. Thus,

many mechanical problems exist and expensive equipment is required formechanical pressing of metal powders.

Another factor of great importance lies in the fact that the resistanceof the powder to compression increases rapidly with the depth of thepowder column. Because of inter-particle friction as Well as frictionwith the die walls, a considerable variation of density will occur if anattempt is made to form a compact more than a few inches in depth. In anumber of applications, this limitation presents a very serioushandicap. This is particularly true in the case of the refractory metalswhich must be further processed by a melting operation.

As previously mentioned, the compressed powder must be sintered in'orderto provide sufficient strength for ordinary handling. This sintering maybe performed by external heating or by internal heating of the compactcaused by passage of an electric current through the compact. Thetemperatures involved are slightly lower than the melting temperature ofany of the components of the compact so that the powder particles do notfuse or melt, but the solidity of the compact results from therearrangement of individual particles and their growth at the expense ofneighboring particles. Furthermore, it has been found to be bothdifficult and costly to fabricate certain metals such as titanium andniobium by conventional methods.

The present practice is to produce electrodes by handwelding a number ofcompacts together and then forming an ingot by an arc-melting process inwhich the electrode is consumed. Since pressed compacts are of irregulardensity, the ingot produced usually contains flaws, and the process isrepeated using the first ingot as the electrode in order to provide abillet or slab suitable for fabricating finishedproducts. Also, thedensity of the compact directly affects the quantity of metal in theelec- =trodeif the density is low, a large electrode will produce asmall ingot and several ingots may have to be welded together to producea billet or slab of the desired size. Experience has shown thatcontinuous arc-melting is required to produce a satisfactory billet,therefore a billet of desired size cannot be prepared by melting two ormore unj'oined electrodes one after the other.

The described techniques of powder metallurgy are applied to manymaterials other than pure metals. The preparation of compacts of metalcompounds such as metal oxides, metal carbides, metal nitrides, andmetal borides is widely practiced in view of the growing needs forarticles prepared from such materials. Nonmetal powders such as carbon,silicon, organic compositions, and polymeric materials are alsocompacted by the described techniques. Mixtures of metals and othermaterials such as binders, lubricants, etc., are compacted to providecompositions having special properties. The so-called selflubricatingbearing is an exampleof this type of compact. To a greater or lesserdegree, the problems of pressing these materials are as severe as arethose for the metal powders.

From the foregoing, the inadequancy of the present procedures isapparent. The preparation of small compacts by pressing and theirjoining by handwelding is very time-consuming and uneconomical. Thelow-density compacts obtained by the conventional pressing methodsrequire excessive further processing, thus adding still more to the timeand expense required to convert the metal powder into a fully usableform. Thus the need for a better method of preparing metal compacts isobvious.

In US. Pat. 2,648,125, a method of preparing compacts is described whichinvolves loading the powder into an impervious bag, immersingv thefilled bag in water contained within a heavy walled vessel, and thencompacting the powder by the hydrostatic pressure produced by the actionof a piston on the body of water, the piston being driven by the gasesproduced from a deflagrating explosive charge. According to the patentdescription, uniform pressures of about 50,000 pounds per square inchcan be obtained and a uniform compact is thus produced. In an articlepublished in Business Week (September 20, 1952), the preparation ofcompacts by immersing a bag filled with powder in water contained withina 14- inch cannon breech block and detonating a dynamite charge withinthe sealed breech block is also described. To some degree, the foregoingmethods overcome the disadvantage of mechanical presses by eliminating adie and provid ng a uniform pressure on all sides of the powder beingcompacted. However, the size limitation remains because it is notpracticabl eto build a very large vessel capable of withstandingpressures of 50,000 pounds per square inch, and the maximum pressurewhich can be applied to the compact is limited because of therequirement of a vessel capable of withstanding the pressure produced.

Accordingly, an object of the present invention is to provide a methodfor compacting powders wherein all of the above described disadvantagesare overcome. A further object is to provide a method for preparingcompacts of essentially theoretical density. A still further object isto provide a method for preparing compacts whereby unique results can beobtained. Additional objects will become apparent as this invention ismore fully described.

We have found that the foregoing objects are achieved when we prepare acompact by surrounding a mass of the I powder with a layer of ahigh-velocity detonating explosive and then detonate the explosivecharge. In order to obtain uniform compaction of the powder, the layermust have a substantially uniform quantity of detonating explosive perunit of area. Furthermore, if the explosive layer is initiated at onepoint only, shock waves converging at the surface 180 from the point ofinitiation will be produced. The higher pressures at the convergingfront introduce stresses in the compact which may cause cracking.Accordingly, therefore, we prefer to initiate the explosive layer at asuflicient number of locations to avoid the production of convergingshock waves. The most satisfactory initiation is provided when theentire periphery at one end of the layer is simulaneously initiated.

For ease of handling and to prevent loss of material, the powder to becompacted is preferably encased in a container. Inasmuch as a seam of acontainer represents a discontinuity in its surface, such scam willintroduce a defect on this surface of the metal compact; thus, we preferto use a seamless container. By using a container of sufficiently heavywalls, wrinkling of the container and consequent surface irregularitiesof the compact can be avoided-the container will be uniformly compressedto compensate for the reduction in volume accompanying the compaction ofthe powder. In order to eliminate the need for removing the containerafter compaction, a container fabricated from the same composition asthe powder being compacted may be used.

In order to illustrate the carrying out of the method of the presentinvention, reference is now made to the accompanying drawings in whichFIGURE 1 is a sectional lengthwise view of an assembly for compactingpowder, and FIGURE 2 is an end view of the same assembly and FIGURE 3 isa side view partially in cross section illustrating further embodimentsof the method of the present invention. In FIGURE 1, 1 represents a massof powder within tube 2 fastened at one end to base 3 and closed at theother end with stopper 4. 5 represents a wrap of a high velocitydetonating explosive around tube 2, 6 is a layer of a detonatingexplosive around inert cone 9, and 7 is a conventional blasting caphaving electrical lead wires 8.

In carrying out the method of the present invention,

when cap 7 is actuated, explosive layer 6 is initiated. Because of theconical configuration of layer 6, explosive wrap 5 is initiatedsimultaneously all along its edge adjacent to layer 6, and thedetonation proceeds along the length of wrap 5 without the formation ofany undesirable converging shock waves. The detonation of wrap 5constricts tube 2 and compresses powder 1.

FIGURE 3 illustrates the interposition of a water annulus between thepowder to be compacted and the layer of high explosive and also thesubmergence of the compaction assembly in an unconfined 'body of a densefluid, for example water. In this figure, elements 1 to 9 are as inFIGURES l and 2. In accordance with one embodiment of the inventionillustrated in FIGURE 3, explosive layer 5 surrounds a retainer element,e.g. a can, T10, which maintains the water annulus 11 between the massof powder I and explosive layer 5, retainer 10 being fastened, eg bywelding, to base plate 3. Conical stopper 9, which is surrounded byexplosive layer 6, is fastened to retainer 10. In accordance with afurther embodiment of the invention, the assembly is shown submerged inan unconfined body of water, the surface of which is indicated by 12.The assembly is suspended in the water by suspending-retaining means 13,shown as a chain attached to an eye hook fastened to base plate 3.Optionally, channels 14 may be provided, as shown, in base 3 when theassembly of the water-annulus technique is to be submerged in water, topermit filling of the space between powder container 2 andwater-retainer 10 with water. 0

In order to illustrate further the present invention, reference is nowmade to the following examples. In the examples, unless otherwiseindicated, the explosive wrap consisted of sheet explosive prepared byblending parts of PETN, 7.5 parts of butyl rubber, and 7.5 parts of athermoplastic terpene resin (mixture of polymers of )3- pinene havingthe formula (O l-1 commercially available as Piccolyte S-1O manufacturedby the Pennsyl- Vania Industrial Chemical Corporation), and rolling theblend into sheets, the thickness of the sheet determining the weight perunit of area. The composition has a velocity of detonation of about 7200meters per second and the sheets are strong, flexible, and nonresilient.

Example 1 A seamless tube of cold-drawn steel having a length of 8inches, an inner diameter of 1% inches, and a wall thickness of A inchmounted on a steel base plate 3 inch x 3 inch x /2 inch was filled towithin /2 inch of the top with titanium sponge, the tube being vibratedduring loading. The titanium sponge thus packed had a bulk density ofabout 1.5 grams per cubic centimeter. A rubber stopper inch in lengthwas inserted in the open end, and a clay cone 1% inch in length wasplaced on the stopper. The tube was wrapped with the described explosivesheet, the explosive load being 2 grams per square inch. A sheet of thesame compo tion but containing 4 grams of explosive per square inch wasused to form the layer about the cone, and this layer was joined to thewrapping around the tube by taping. A commercial detonator (a number 8electric blasting cap) was fastened to the apex of the conical layer,and the assembly was immersed in water. The blasting cap was initiatedby application of an electric current, causing detonation of both layersof explosive. The assembly was recovered and the constricted outer tubewas re moved.

The compact produced was very strong, had the appearance of a solidmetal rod, a diameter of 1 inch, and a density of 4.36 grams per cubiccentimeter, 97% of the theoretical density.

Example 2 The procedure of Example 1 was repeated using dendritictitanium powder in place of the titanium sponge.

pearance of a solid metal rod.

Example 3 The procedure of Example 1 was followed by using atomizedaluminum powder (60% through a 325-mesh screen) packed in a stainlesssteel seamless tube 6 inches in length, 2% inches in inner diameter, and1 inch in wall thickness. The packed powder had a bulk density of about1.5 grams per cubic centimeter. The compact produced had a density of2.65, 98% of theoretical density, and was completely free of cracks. Thediameter of the compact was 1 inches. A transverse slab cut from thecompact was used to determine tensile strength and gave a measurement of5,400 pounds per square inch.

Example 4 The procedure described in Example 1 Was used to compactcopper powder having a bulk density of 4.1 grams per cubic centimeter ina cast iron tube 1 1 inches in inner diameter, 7 inches in length, and/3 inch in wall thickness by using an explosive load of 3 grams persquare inch. The compact produced had a density 97% of theoretical (8.92grams per cubic centimeter) and a diameter of i inch. The center wasunconsolidated, the diameter of the unconsolidated portion being about0.05 inch.

Example The procedure described in Example 1 was used to compactelectrolytic iron powder, which had been vibratorpacked into thecompaction tube to a bulk density of about 3.8 grams per cubiccentimeter. The results are as indicated in the following table.

ODS-cold-drawn; SS-staiuless steel; EA-extruded aluminum.

Example 6 The procedure of Example 1 was followed to compact niobiumpowder (through 30-rnesh, held on SO-mesh screen) in 21 37 /22 inch longtube of cold-drawn steel having an inner diameter of 1% inches and awall thickness of V inch, the explosive loading being 5 grams per squareinch. i The diameter of the compact was 1 inch, and the density wasgreater than 95% of theoretical. The compact was sufiiciently strong tobe used directly as a consumable electrode for the further processing ofthe niobium.

This particular powder could not be consolidated by mechanical pressesapplying up to 30 tons per square inch.

Higher mechanical pressures were not feasible because of the excessivewear on the dies due to the hardness and abrasiveness of the niobiumparticles.

Example 7 8 inches. in length, 1% inches in, inner diameter, and

inch in wall thickness. The results obtained were as follows:

Compact Compact Density (g.lcc.) explosive diameter loading (inches)(g./sq. in.) Peripheral Center A 2 1% 8. 8 7. G 4 1%2 9. 6 8. 1 C 8 Hot10. 0 9. 6

The theoretical density of uranium oxide is 10.9 grams per cubiccentimeter.

Compact A had the brown color of the uncornpacted powder, approximatelya inch diameter central portion was unconsolidated; compact B was alsobrown and had a center about /9 inch in diameter of unconsolidatedpowder; compact C had a black, metallic color and no unconsolidatedportion.

Example 8 This example illustrates the preparation of a berylliumcompact following the procedure of Example 1. Beryllium powder, particlesize about 50 microns,when vibrator packed to a density of about 0.6gram per cubic centimeter in a seamless steel tube 8 inches in length,2% inches in diameter, and V inch in wall thickness, and compressed bythe detonation of an explosive loading of /a gram per square inch,undergoes a reduction to approximately /3 its original volume, i.e., toa diameter of 1 inch, resulting in a coherent compact having a densityapproximately of theoretical.

Example 9 Tungsten powder was vibrator packed to a density of 7.3 gramsper cubic centimeter in a cold-drawn seamless steel tube 1% inches ininner diameter, inch in wall thickness, and 8 inches in length. A sheetof explosive having a loading of 8 grams per square inch was wrappedaround the tube, and the procedure described in Example 1 was followedto compact the powder. The diameter of the compact obtained was inch andthe compact was at 97.82% of theoretical density (19.3 grams per cubiccentimeter). The compact had sufiicient strength to withstand ordinaryhandling.

Example 10 Example 11 i Silicon, obtained in the form of needles, wasball-milled and vibrator packed into a 2% inch inner diameter steel tube8 inches in length and ,5 inch in wall thickness. Following theprocedure of Example 1, using an explosive 7 loading of 2% grams persquare inch, a compact having a diameter of 2%; inches was obtained. Thedensity was 95% of theoretical (2.4 grams per cubic centimeter).

Example 12 Dendritic chromium powder was compacted to 95% of theoreticaldensity (7.1 grams per cubic centimeter) by the procedure described inExample 1. The chromium powder, vibrator packed to a density of 1.8grams per cubic centimeter in a seamless steel tube having an innerdiameter of 1% inches, a wall thickness of A inch and a length of 8inches was compacted using an explosive loading of 3' grams per'squareinch to a diameter of 1 ,4 inch.

Example 13' A stainless steel tube 8 inches in length, 2% inches ininner diameter, and inch in wall thickness was packed 7 with 3 /2 inchesof copper powder and 3 /2 inches of atomized aluminum powder. Theprocedure described in Example 1, an explosive loading 2 grams persquare inch being used, was followed to form the compact. The compactedaluminum and copper were at 98% of their theoretical density, and thetwo metals were firmly joined to each other.

Example 14 A powder mixture consisting of 60 parts by weight ofelectrolytic iron, 30 parts of atomized aluminum, and 10 parts of copperwas compacted by the procedure of Example l in a stainless steel tube 2%inches in inner diameter, 1 inch in wall thickness, and 8 inches inlength by using an explosive loading of 4 grams per square inch. Thecompact had a diameter of 1%; inches.

Example 15 The procedure used in Example 1 was followed to prepare acompact of a steel alloy powder commercially sold for use in powderfabrication. The alloy has a composition of 0.5%carbon, 0.35% silicon,0.75% manganese, 0.010% sulphur, 0.01% phosphorus, 1.85% nickel, 0.25%molybdenum, and the balance iron. In normal powder metallurgy practice,this powder is compacted at 30-50 tons per square inch and sintered at2000 to 2100 F. To obtain greater strength and ductility, the sinteredparts are reprocessed by sizing and resintering. This procedure shouldresult in a compact having approximately 95% of theoretical density.

When compacted by an explosive loading of grams per square inch in a 1%inch inner diameter, 6-inch-long cold-drawn steel seamless tubing havinga A inch wall thickness, the compact produced had a diameter of 1% inchand a density 97% of theoretical without sintering.

Example 16 A compact consisting of an iron core surrounded by analuminum sheath was prepared by centering a thin-walled tube of Vz-inchdiameter with a cold-drawn steel seamless tube 1% inches in innerdiameter inch in wall thickness), filling the thin-walled tube withelectrolytic iron powder and the annular space around the thin-walledtube with aluminum powder the tubes being vibrated during the fillingoperation. The thin-walled tube was then removed, and the assemblycompacted, by using explosive sheet at a loading of 2 grams per squareinch. The compact was free of cracks, had a diameter of 1% inch, and avery strong bond existed between the two metals at their interface.

Example 17 The procedure of Example 1 was followed using a powderedniobium alloy composition consisting of 75% niobium, 15% molybdenum, andtitanium. Satisfactory compacting was obtained.

The foregoing examples illustrate the application of the method of thisinvention to a variety of powders. In essence, the present method canadvantageously be used in the compacting of any powder that can bemechanically compacted, and to powders that cannot be mechanicallycompacted. Because the explosive sheath can be made as long as desired,no limit exists with respect to the length of the compact which can bethus produced. As shown by the examples, excellent iron compacts of 5 /2inches in diameter were prepared. There is no reason to believe thatsuch diameter represents nearly the maximum diameter compact which canbe produced, particularly since an explosive loading of only 16 gramsper square inch was required. The examples further show that thecompacts obtained are of much higher density than that obtainable byconventional procedures, even including sintering. Inasmuch as thestrength and working characteristics of a compact are directly relatedto its density, the obtaining of such high densities in a singleoperation is of tremendous significance. For many fabrications,

the compact thus formed can be used without further processing. Whenfurther processing is desired, such as for example in preparing billetsfrom the compact by the consumable electrode process, the quantity ofmaterial in an electrode of a specified size is greatly increased by thehigher density, and the higher strength and uniform distribution of thematerial causes a more regular and uniform melting zone. This latterfeature may permit elimination of a second remelting operation in orderto obtain a uniform billet.

The very high densities, usually almost theoretical, of the compacts arebelieved due to the tremendous pressures produced by the detonation waveitself acting on the powder. Calculations have indicated that thepressure of a detonation wave at the surface of a high-velocitydetonating explosive charge are somewhere near 200,000 atmospheres. Asthe detonation wave progresses outwardly from the explosive charge, itnormally is rapidly attenuated and the pressure drops rapidly. Forexample, if a spherical charge is centrally initiated, the shock waveproduced issues as a spherical wave. The area of the sphere increases asthe square of the radius, and therefore, the pressure mustcorrespondingly decrease. However, when the explosive charge surroundsthe object, the pressures directed toward the center are reinforcedrather than attenuated. Thus, while the compaction of the metal absorbsenergy rapidly, the concentration of the shock wave towards the centerprovides the energy needed to compact beyond the surface.

That the radially symmetric application of the pressure of thedetonation wave is responsible for the high density obtained is fullyillustrated by the following comparative examples.

Example 18 Two aluminum electric blasting cap shells (0.260 inch innerdiameter, 0.010 inch wall thickness) were vibrator packed withelectrolytic iron powder and the end of the shell sealed by crimping ina rubber plug. One shell was wrapped with explosive sheet in accordancewith the present invention, the total explosive load (PETN) being equalto 1.6 grams. The assembly was submerged in an open body of water andthe explosive sheet initiated by means of a No. 6 commercial electricblasting cap. The compact produced had a density of 7.68 grams per cubiccentimeter, 97.5% of theoretical.

The second shell was placed in a bomb, a quantity of the same sheetequal to 1.6 grams of explosive and a No. 6 electric blasting cap wereplaced together in the bomb, and the bomb was filled with water andsealed shut by a rubber-gasketed cap plate bolted in place, provisionhaving been made for the passage of the lead wires to the outside of thebomb. The cap was initiated, in turn detonating the PETN sheet. Thecompact thus formed had a density of only 6.23 grams per cubiccentimeter, i.e., 79.5% of theoretical density.

The foregoing indicates clearly that the unfocused hydrostatic pressureproduced by the detonation of an explosive in a confined body of wateris much less than the pressure exerted by the same amount of explosivewhen the explosive surrounds the material being compacted. In theforegoing experiments, only very small compacts were prepared because ofthe requirement of a bomb having sufiicient strength to withstand thepressures produced by even the small quantity of explosive used. Thehydrostatic pressures on the walls of the bomb (as well as on the wallsof the shell containing the compact) were about 50,000 pounds per squareinch.

The beneficial effect of focusing the pressure produced by thedetonation of the explosive charge does not depend upon the explosivebeing in contact with or even adjacent to the material being compacted.The following example illustrates the foregoing.

Example 19 Tungsten powder was vibrator packed to a density of ab ut -3r m P cub c ce t me er a c ds am s s e tub 1% ine es in i ner d eter hech in wall thickness, and 8 inches in length. The bottom of the tube waswelded to a &1 inch thick steel disk 4 inches in diameter, and the topwas sealed with a rubber stoppet. The assembly was then placed in a canhaving a diameter of 4 inches and a wall thickness of &4 inch. Theannular space between the can and the tube was filled h Wa e and he em!of h can a l ed by a conic stopper, Sheet explosive having an explosivecontent of 4 grams per square inch was wrapped around the can and cone,the assembly was submerged in water and the explosive was initiated atthe apex of the cone by a No. 8 commercial blasting cap,

The compact produced had a diameter of inch and 98% of theoreticaldensity. The compact was free of cracks and of suiiicient strength forrough handling.

However, if desired, the explosive may be in contact with the powderbeing compacted. The following exam; ple illustrates such procedure.

Example 20 A sheet of explosive having an explosive loading of 2 gramsper square inch was fitted inside a steel tube 1% inch in innerdiameter, inch in wall thickness and 8 inches in length so that theinner surface of the tube was covered at all points by the explosivesheet. Iron powder was then vibrator packed into the explosive-linedtube to a bulk density of about 3.8 grams per cc. The top of the tubewas sealed with a conical rubber plug and a layer of 4 grams per squareinch explosive sheet wrapped around the coneand about, /2 inch of thetube adjacent to the cone. The assembly was submerged in water and theconical explosive layer initiated with a N0. 8 electric blasting cap.The detonation of the layer overlapping the tube initiated the explosivewithin the tube. The tube itself was shattered, but a compact of ironpowder having a diameter of 1 inch and a density 95% of theoretical wasrecovered. The compact showed a surface irregularity corresponding tothe line formed by the meeting edges of the sheet explosive-suchirregularity could be eliminated by using extruded explosive tubeinstead of sheet explosive. In order to provide the high pressuresrequired for compaction in accordance with the present invention, theexplosive must detonate at high velocity. An unconfined defiagratingexplosive could not provide the desired pressures. By the termhigh-velocity detonating explosive, We mean a composition having, whenunconfined, a velocity of detonation of at least 1200 meters per second.The selection of the particular high-velocity detonating explosive to beused will be dependent upon the size of the compact to be. prepared.Obviously, the quantity of explosive in the layer surrounding thecompact must be suiiicient so that the detonation will be propagatedthroughout the. entire. length of the layer. In the case ofsmall-diameter compacts, the quantity of explosive per unit of arearequired is so small that the less sensitive explosives such as RDX,i-IMX, TNT and the like would not propagate the detonation. On the otherhand, the more sensitive explosives, such as PBTN and nitroglycerinbasedexplosives, which will propagate detonation even in a thin layer willalso propagate detonation in thick layers. Thus, when large-diametercompacts are desired, a much wider range of explosive compositions canbe used.

In the preceding examples, a PETN-binder composition formed as aflexible sheet was used to Provide the compacting energy. This form ofexplosive represents a preferred form because of the ease of applicationand control of loading density. The following example illustrates theuse of biasing gelatin (91% nitroglycerin, 8% nitrocellulose, 1% chalk).

Example 21 A cold-drawn steel seamless tube 1% inch in inner nessmounted on a 3 x 3 x /2 inch base plate was vibratorpacked withelectrolytic iron powder, and the open end was sealed with a rubberstopper. A clay cone was taped to the tube, and the surface of the tubeand clay cone coated with a flinch-thick layer of blasting gelatin(about 7 grams per square inch). The, blasting gelatin was initiated atthe cone apex by means of a No. 6 electric blasting cap. The compactproduced had a diameter of 13/ inch and was at, 97.7% of theoreticaldensity. This explosive loading represents a considerable overloadingand the effect of the overloading was to produce a fused area in thecenter of the compact due to the concentration of the shock waves, thisfused area resulting in a small axial channel.

As previously mentioned, a compact having a good appearance can beobtained only by initiating the explosive layer in such a way that theshock waves from the detonation converge only at the center of thecompact. If only one point on the periphery of the explosive layer isinitiated, the detonation will be propagated. around the periphery aswell as axially, and the detonation fronts will converge in a line fromthe point of initiation. This convergence of detonation waves willproduc pressures along this line much greater than those elsewhere onthe surface of the compact causing distortion, if not cracking. Further,the density within the compact will not be uniform in the absence of thecentrally converging shock wave. As shown by the procedure described,the. formation of the undesired converging shock wave can be avoided byinitiating the entire edge simultaneously. In the present examples, aconical charge initiated at the apex was used. Other means of producinga linear shock wave can also be used. If desired, a flat Wafer initiatedat the center could be employed. If a conical charge is used, theinterior of the cone should be filled with an inert material to preventthe formation of a jet which would damage the end of the compact.

In order to reduce noise and air blast from the detonation of theexplosive layer, we prefer to submerge the assembly in water prior toinitiating the explosive. Inasmuch as the water is not required totransmit the pressure from the detonation of the explosive charge, noconfinement of the water is required. The explosive compositions usedwhen the assembly is submer ed must obviously be water-resistant. Tofacilitate recovery of the compact, we prefer to attach'a retainingmeans, such as a cable or chain, to the base plate of the assembly. Ifthe assembly is initiated in air, no retaining means is needed toprevent the compact from'being thrown bythe blast.

The. following example illustrates the compaction of powder in air andalso the eficct of the explosive loading on the compaction.

Example 22 lytic iron and was vibrator packed to a density of about 3.8grams per cubic centimeter in the tube.

Ralo 0t Explosive explosive Compact Compec Tube loading weight todiameter tion ratio (ax/sq. in.) powder (in) weight 1 Excluding theconical explosive layer.

The surfaces of all compacts had a smooth metallic appearance. Compact Ahad an unconsolidated center 1% inches in diameter, but the tubularshell of consolidated iron provided excellent rigidity. Compact B had anunconsolidated center ,5 inch in diameter, and compact C had anunconsolidated center inch in diameter. Both compact B and compact C hadexcellent strength and an average density of more than 90% oftheoretical. As may be seen by reference to Example 5, when a similarcompaction was made under water of iron powder in a tube having the samediameter and wall thickness, and using 4 grams of explosive per squareinch, a density equal to 98% of theoretical was obtained. This higherdensity is apparently due to the confinement provided by the watersurrounding the explosive charge rather than to any transmission ofpressure through the water. Thus, when the compaction is made in adenser medium, a lower ratio of explosive to powder can be used thanwhen the compaction is carried out in a less dense medium. The presenceof a surrounding relatively dense medium, such as water, also serves toreduce the likelihood of the compact being shattered as a result ofrarefaction due to the great diiference in the velocity of sound in thetwo media.

The compacts produced by the method of the present invention have beenshown to have considerably higher densities than those obtained byconventional pressing methods. For example, using electrolytic ironcontaining 1% by weight of zinc stearate as a lubricant, published dataindicate the following densities can be expected.

Pressure applied (lb./sq. in.)

Density (s/ As shown by Examples 5 and 13, the densities obtained by thepresent method are 7.6 grams per cubic centimeter and higher. Publisheddata show that for the same powder at a green (unsintered) density of6.0 grams per cubic centimeter, a maximum tensile strength of 3800pounds per square inch can be expected, and for a sintered compact ofthis density, a maximum strength of 19,000 pounds per square inch can beobtained. A tensile strength specimen was taken from the compactdescribed in Example 13 and a measurement of 13,600 pounds per squareinch was obtained. The foregoing shows conclusively the superiority inboth density and strength of the method of the present invention overprior methods of compacting powders.

The present method of compacting powders is particularly applicable tothe preparation of compacts of the refractory metals such as titaniumand niobium, and for the compacting of powder compositions having hardand/or abrasive particles. Powders which have been compacted by thepresent method in addition to those shown in the examples include ironoxide, ferrosilicon, sodium chloride, lead dioxide, red lead, alundumand carborundum. As previously indicated, the powders may be compactedin a tube of the same composition as the powders, thus eliminating theneed for removal of the tube from the compacted powder. To providelinear strength, wires or rods of the material being compacted may bepositioned in the powder prior to compaction; such wires and rods willthen reinforce the compact without providing an impurity.

in those cases where the presence of air or other gas may be deleteriousto the powder being compacted, the tube assembly may be evacuated priorto detonation of the explosive layer. If only air is objectionable, theair may be replaced with an inert gas. Obviously, a gas which wouldimpart desirable characteristics to the powder may be included in thetube.

If desired, the powder may be heated prior to being compacted. Inductionheating can be used to heat powders in the tube, and an explosive havinghigh thermal stability can be used. Similarly, a magnetic field can beimpressed on the powder to orient magnetic material in the powder priorto compaction.

The invention has been described in detail in the foregoing. Manymodifications and variations that lie within the scope of this inventionwill occur to those skilled in the art. Accordingly, we intend to belimited only by the following claims.

We claim:

1. A process for compacting powder which comprises surrounding a mass ofpowder with a layer of high-velocity detonating explosive, said layerbeing substantially continuous and uniform in composition, andinitiating the entire periphery of one end of said explosive layersubstantially simultaneously.

2. A process for compacting powder which comprises surrounding a tubularcontainer of a mass of powder with a layer of high-velocity detonatingexplosive, said explosive layer having a substantially continuous anduniform composition, and positioning a detonating explosive having aconical configuration with the base of said cone adjacent to one edge ofsaid layer of explosive, and thereafter initiating said conicalexplosive at its apex.

References Cited in the file of this patent UNITED STATES PATENTS2,648,125 McKenna et al Aug. 11, 1953 2,711,009 Redmond et a1 June 21,1955 2,781,273 Koch Feb. 12, 1957 2,783,504 Hamjian et al. Mar. 5, 1957

1. A PROCESS FOR COMPACTING POWDER WHICH COMPRISES SURROUNDING A MASS OFPOWDER WITH A LAYER OF HIGH-VELOCITY DETONATING EXPLOSIVE, SAID LAYERBEING SUBSTANTIALLY CONTINUOUS AND UNIFORM IN COMPOSITION, ANDINITIATING THE